This invention relates to a semiconductor device. More particularly, this invention relates to a semiconductor device containing a high integration density memory using memory cells for storing information by utilizing a change of a magnetic resistance.
A magneto-resistive random access memory (MRAM) has been developed as a memory that has no limitation to the number of times of read/write operations though it is one of the nonvolatile memories typified by a ferro-dielectric memory (FeRAM) and a flash memory. This MRAM stores information by utilizing a magneto-resistive effect in which a resistance of a device varies depending on a direction of magnetization. Development of a magnetic tunnel junction (MTJ) device of which magneto-resistance change ratio referred to as “MR” is greater than those of conventional devices and its application to the MRAM have been carried out in recent years, and such studies have revealed the possibility that a high-speed write operation comparable to that of a static random access memory (SRAM) and a high integration density comparable to that of a DRAM can be achieved. Therefore, the MRAM has now drawn an increasing attention as a promising applicant of the next generation memories.
The MTJ device has a three-layered structure in which an insulating film is sandwiched between two ferromagnetic layers FRL and FXL as shown in FIG. 3 of the accompanying drawings. The insulating film TB is formed to a small thickness such that electrons can be transferred by a tunnel effect. The direction of magnetization of the ferromagnetic layer FXL is fixed as represented by an arrow MAF2 whereas the direction of magnetization of the ferromagnetic layer FRL is controlled by an external magnetic field as represented by an arrow AMF1. The resistance between terminals A and B varies depending on the directions of magnetization in these two ferromagnetic layers. The resistance is low when the directions of magnetization are the same and is high when they are opposite. An MRAM to which such an MTJ device is applied is described, for example, in IEEE International Solid-State Circuits Conference, DIGEST OF TECHNICAL PAPERS, pp. 128-129, 2000 (hereinafter called the “cited reference 1”), and in the same DIGEST OF TECHNICAL PAPERS, pp. 130-131 (hereinafter called the “reference No. 2”). In either case, a construction in which one MTJ device and one transistor are connected in series constitutes a basic construction of a memory cell. When the transistor in a selected memory cell is conductive, a voltage is applied across both terminals of the MTJ device, and memory information is read out by detecting a current that flows through a data line in accordance with the magneto-resistance.
FIG. 4 shows a current that develops when a voltage is applied across both terminals of the MTJ device at a time T1. It will be assumed hereby that the resistance state is high when the MTJ device holds the memory information ‘0’ and is low when it holds the memory information ‘1’. At this time, a current ID(1) obtained by reading the memory cell holding the memory information ‘1’ is greater than a current ID(0) obtained by reading the memory cell holding the memory information ‘0’, and both currents assume positive values. Owing to such characteristics of the MTJ device, the following two problems develop in the read operation of the MRAM. First, a reference signal is necessary for discriminating the memory information from the read signal having one of the polarities. Second, because the MR of the MTJ is only dozens of percents, a read signal quantity is small and a stable read operation is difficult.
To solve these problems, the cited reference 1 employs a twin cell system using two MTJ devices and two transistors to constitute the memory cell. This system can acquire complementary read signals in accordance with the memory information of the memory cell. Therefore, discrimination of the information is easy and the signal quantity is great, too. However, because the memory cell area becomes twice, this system may be relatively disadvantageous for attaining a large capacity. In contrast, the cited reference No. 2 generates the reference signal by arranging a reference cell composed of one MTJ device and one transistor, each being the same as that of the memory cell, for each word line. This system can restrict the memory array area. However, it may presumably be difficult to form the reference cell that precisely generates the reference signal while variance of characteristics of each memory cell is taken into account. When any defect such as disconnection or short-circuit occurs in the reference cell or in data lines to which the reference cell is connected, the reference signal is not generated with the result that the memory information of a plurality of memory cells cannot be read out and a drop of yield is likely to occur. The present invention is completed on the basis of the examination result described above.